Steady efforts have been focused on the development of hybrid vehicles that travel based upon the outputs from both an engine and a rotating electric machine and pure electric vehicles that travel exclusively based upon the output torque provided by a rotating electric machine. In this specification, the term “electric vehicle” is used to refer to both a hybrid vehicle and a pure electric vehicle. The rotating electric machine installed in an electric vehicle to enable traveling operation, which is required to provide a large output, includes permanent magnets constituted with sintered rare-earth magnets having strong magnetic energy. In the rotating electric machine installed in the electric vehicle to propel the electric vehicle, the permanent magnets are embedded within a rotor core.
Torque pulsations, induced by a cogging torque or the like, are bound to occur in a rotating electric machine. Such torque pulsations occurring in the rotating electric machine may cause noise or vibration. Torque pulsations occurring in an electric vehicle, in particular, may cause driver/passenger discomfort during vehicle takeoff. Accordingly, various attempts have been made in the related art to reduce torque pulsations in rotating electric machines by slightly changing each position of the permanent magnets at the rotor along the circumferential direction according to the axial direction. Such technologies are taught in patent literature 1 and patent literature 2.
In the rotating electric machine described in patent literature 1, the position of each permanent magnet embedded in the rotor core is slightly offset along the circumferential direction in correspondence to the position of the permanent magnet assumed along the rotational axis of the rotating electric machine. The publication discloses a technology for reducing torque pulsations by slightly offsetting the position of the permanent magnet along the circumferential direction in correspondence to its position assumed along the rotational axis as described above.
The rotating electric machine used in automotive applications typically adopts a structure that is bound to induce a significant reluctance torque in order to minimize the extent to which the torque output is reduced in a high-speed rotation range. In other words, its rotor adopts a structure that assures a lower magnetic resistance at auxiliary magnetic poles through which a q-axis magnetic flux passes, relative to the q-axis magnetic flux generated with an electric current flowing through the stator, in order to increase the reluctance torque. This rotor structure requires less magnet mass and makes it possible to keep down the internally induced voltage in the high-speed rotation range, since the increased reluctance torque reduces the ratio of the magnetic torque attributable to the permanent magnets, to the output torque of the rotating electric machine. It is desirable that reluctance torque pulsations, as well as the magnetic torque pulsations, be minimized in such a rotating electric machine in which the reluctance torque is actively utilized.
The rotating electric machine disclosed in patent literature 1 does not adopt a structure that actively utilizes reluctance. Accordingly, patent literature 1, which simply refers to torque pulsations of the magnetic torque, does not mention or imply in any way whatsoever torque pulsations attributable to the reluctance torque. In addition, the torque pulsations described in patent literature 1, which contains a significant axial component, manifesting along the axial direction in correspondence to the torque generated at the rotating electric machine, give rise to a problem in that a large thrusts force is bound to occur along the axial direction in correspondence to the torque generated at the rotating electric machine.
While reluctance is actively utilized in the structure adopted in the rotating electric machine disclosed in patent literature 2, the publication does not discuss torque pulsations attributable to the reluctance torque.